In an optical communication, pulse-amplitude modulation (PAM) encodes information in the amplitude of a series of optical signal pulse. With the increasing demand for more efficient ways to transmit optical signals, some optical networks use multi-level PAM such as PAM-8, 16, 32 or higher, which can have multiple distinct amplitude levels. Optical devices that employ such a high-order PAM, however, may exhibit nonlinear effects or saturation effects, which lead to a degradation of the quality of the optical signal.